I don't have to, I just want to.

No one has responded yet, so even as a native speaker I'll comment.

I'm always trying to improve my English, because I think it's important to be articulate and to sound intelligent. It's much easier to communicate if you can speak precisely. 

Every time i went ON foreign countries i couldn't comunicate. It's umiliating. More than, i'm studying to grow up my skills

Because I chose the States as a place to work. 

I have to improve my English, skills especially speaking. 

If I were in Japan and only friends with Japanese, yes I don't need to study English. However my friends are not only Japanese, so if I'd like to communicate with my friends without any problems, I have to understand the language, English. 

because, in my country, we need english to read foreign's report and communicate with foreign specialist. Now, many doccument is wrote by english. so I think I must learn english